1. Field of the Invention
The present invention relates to a signal processing apparatus arranged to amplify a predetermined frequency component of a signal on the basis of a predetermined amplification characteristic prior to the transmission of the signal and to recover the original signal after the reception thereof.
2. Description of the Related Art
Various kinds of signal processing apparatus have heretofore been provided for the purpose of communicating information signals through transmission and reception thereof or by recording the information signals on a recording medium and reproducing the same therefrom.
As an example, a so-called still video apparatus is known which is arranged to record still image signals on a recording medium such as a magnetic disc, and reproduce the same therefrom, by means of a magnetic head. In recent years, as video tape recorders (VTRs) or other video equipment has been enhanced in image quality, there has arisen a strong demand for an improvement in the image quality of such a still video apparatus.
FIG. 1(a) shows the frequency allocation of a recorded signal which is formed by a current type of still video apparatus. As illustrated, a luminance signal which contains a synchronizing signal is frequency modulated into a high-frequency band signal (represented as YN in the figure), while a chrominance signal is frequency-modulated into a low-frequency band signal (represented as C in the figure) as a known color-difference line-sequential signal. Thus, the luminance signal and the chrominance signal are frequency-multiplexed to form the above-described recorded signal. In this case, the luminance signal is frequency-modulated such that the frequency f.sub.1 of its sync. tip and the frequency f.sub.2 of its white peak are 6.0 MHz and 7.5 MHz, respectively.
On the other hand, a so-called high-band system using an enhanced carrier frequency of a frequency-modulated luminance signal has been proposed as one method of enhancing image quality. As shown in FIG. 1(b), in the frequency allocation of a recorded signal which is formed by such a high-band system, its high-frequency end extends into a band which is high compared with that of the conventional system shown in FIG. 1(a). In accordance with the high-band system, as shown in FIG. 1(b), its luminance signal (represented as Y.sub.H in the figure) is frequency-modulated such that the frequency f.sub.3 of its sync. tip and the frequency f.sub.4 of its white peak are 8.0 MHz and 9.5 MHz, respectively.
However, if a recorded signal according to the above-described high-band system is to be formed by using a magnetic head and a magnetic disc which have the same construction as conventional ones, then the higher the frequency of the recorded signal, the lower the signal level of the same due to the electromagnetic conversion characteristics. Accordingly, to improve the S/N ratio of a reproduced signal, it becomes necessary to increase the amount of emphasis which has conventionally been affected on luminance signals.
FIG. 2 diagrammatically shows the essential construction of an apparatus for recording and reproducing still image signals in accordance with the above-described high-band system.
As illustrated, a luminance signal which contains a synchronizing signal is supplied to an input terminal 1, clamped, for example, at its sync. tip by a clamping circuit 2, and input to a non-linear emphasis circuit 3, in which the signal is subjected to non-linear emphasis. Further, the non-linearly emphasized signal is input to a linear emphasis circuit 4, in which the amount of emphasis applied to the signal is increased. Then, the signal is frequency-modulated in a frequency modulator 5, passed through a recording amplifier 6 and a switch 7, and recorded on a magnetic disc 9 by means of a magnetic head 8. During recording, the magnetic disc 9 is rotated once for each field period by means of a motor 10. Accordingly, still image signals corresponding to one field are recorded on each track which is formed concentrically on the magnetic disc 9.
In addition, since the magnetic head 8 is capable of traveling over the magnetic disc 9 in the radial direction thereof, still image signals for some tens of field images can be recorded over the magnetic disc 9.
During reproduction, a small reproduced signal which has been picked up by the magnetic head 8 is passed through the switch 7, amplified to a sufficient level by a pre-amplifier 11, and frequency-demodulated by a frequency demodulator 12. Then, the frequency-demodulated signal is passed through a linear de-emphasis circuit 13 which has the characteristics reverse to those of the linear emphasis circuit 4 so that the portion of that signal which has been emphasized by the linear emphasis circuit 4 during recording is suppressed. Subsequently, the signal is passed through a non-linear de emphasis circuit 14 which has the characteristics reverse to those of the non-linear emphasis circuit 3 so that the portion of this signal which has been emphasized by the non-linear emphasis circuit 3 during the recording is suppressed. The resulting reproduced luminance signals are provided at an output terminal 15.
FIG. 3 is a block diagram of an example of the construction of the non-linear emphasis circuit 3 shown in FIG. 2. As illustrated, a luminance signal which has been supplied to the input terminal of the non-linear emphasis circuit 3 is transmitted through two separate paths. One of the signals passed through these two paths is applied directly to an adder 19, while the signal which has entered the other path is first passed through a high-pass filter (HPF) 16. A high-frequency band having a desired non-linear characteristic is extracted from the signal by the high-pass filter (HPF) 16 and then input to a compressor circuit 17. The extracted high-frequency band is subjected to compression which is weighted by the compressor circuit 17 in accordance with its input level. Then, the obtained non-linear characteristic is weighted by a multiplication-by-factor circuit 18 and applied to the adder 19, in which this multiplied signal is added to the above-described signal which has been input directly to the adder 19. In this manner, non-linear emphasis is affected on the luminance signal.
It is to be noted that, as shown in FIG. 5, the amount of emphasis varies according to whether or not the level of an input luminance signal is large.
FIG. 4 is a circuit diagram showing an example of a emphasis circuit which includes the non-linear emphasis circuit 3 and the linear emphasis circuit 4.
As illustrated, a luminance signal which has been clamped at the sync. tip of its synchronizing signal is supplied to an input terminal, passed through a high-pass filter constituted by a capacitor C4 and a resistor R12, and amplified by an amplifier of the common-base type constituted by a transistor Q5. A soft limiter circuit constituted by diodes D.sub.1, D.sub.2 and resistors R16, R17, R18, R19 and R20 is connected through a capacitor C5 to the collector of the transistor Q5.
FIG. 6 shows the voltage (V) - current (I) characteristic of the soft limiter circuit. As can be seen from this figure, if the level of an input luminance signal is small, the characteristic is determined by r.sub.1 =R18=R19, whereas, if the level is large, the characteristic is determined by r.sub.1 =R20=R16//R17. In this case, it is common practice that the relationship between r.sub.1 and r.sub.2 is set to r.sub.1 &gt;&gt;r.sub.2. The variation of the characteristic is given by ##EQU1## where is the values of the respective diodes D.sub.1 and D.sub.2 when they are "on".
If the value of V.sub.2 -V.sub.1 is determined by selecting resistors R22 and R23 and the resistors R20 and R22 are selected so that R20/R22=2 may be obtained, it is possible to cancel the drift, due to temperature, of the voltage V.sub.D of each of the diodes D.sub.1 and D.sub.2. The above-described operation of the soft limiter circuit causes the gain provided between the resistor R15 and the soft limiter circuit to vary in accordance with whether the level of an input luminance signal is large or small. The above-described circuit portion serves as the compressor circuit 17.
The input signal thus processed is then input to the emitter of the transistor Q3 through the emitter follower of a transistor Q4, a resistor R10 and a capacitor C3, while the luminance signal at the input terminal is also input to the emitter of the transistor Q3 through a resistor R9. These signals are amplified by the common-base type amplifier constituted by the transistor Q3, passed through the emitter of a transistor Q2, then through the linear emphasis circuit constituted by a capacitor C1 and resistors R2, R3, and developed at an output terminal via the emitter follower of the transistor Q1.
The value of the factor K of the multiplication-by-factor circuit 18 shown in FIG. 3 is determined by, for example, the values of the resistors R12, R15, R10 as well as the values of r.sub.1 and r.sub.2.
FIG. 7 is a block diagram showing one example of the construction of the non-linear de-emphasis circuit 14.
In this example, if the level of a signal applied to the input terminal shown in FIG. 3 is made equal to the level of a signal provided at the output terminal shown in FIG. 7, the transfer functions of the compressor circuits shown in FIGS. 3 and 7 equal each other. Therefore, if the open-loop gain of the differential amplifier 20 shown in FIG. 7 is sufficiently large, it follows that the transfer function of the non-linear de-emphasis circuit 14 is equal to the reciprocal of the transfer function of the non-linear emphasis circuit 3.
FIG. 8 is a circuit diagram showing an example of a de-emphasis circuit which includes the linear de-emphasis circuit 13 as well as the non-linear de-emphasis circuit 14 shown in FIG. 7.
As illustrated, the reproduced luminance signal which has been output from the frequency demodulator 12 is input to the base of a transistor Q7. The input signal is first passed through the linear de-emphasis circuit 13 which is constituted by resistors R26 and R27 as well as a capacitor C7 and which has the characteristics reverse to those of the linear emphasis circuit 4. The signal is then subjected to non-linear de-emphasis in the amplifier constituted by a transistor Q9. In this manner, the original luminance signal is reproduced at the emitter of a transistor Q10. A high-pass filter and a compressor circuit, both of which are completely equivalent to those used in the non-linear emphasis circuit 3 shown in FIG. 3, are connected to the emitter of the transistor Q10, and a signal component having a non-linearly emphasized characteristic is output from the emitter of the transistor Q4.
This signal is inverted by the inverting amplifier constituted by a transistor Q12, input to the base of a transistor Q11, and supplied from the emitter of the transistor Q11 through a resistor R35 to the emitter of a transistor Q9, whereby the feedback loop of the non-linear emphasis portion is formed. The value of the feedback factor is made equal to the value of the factor K used in the non-linear emphasis circuit 3 by selecting the values of, for example, resistors R37, R38 and R35.
In a case where recording based on the high-band system is carried out, the amount of emphasis is increased as described previously. However, it is common practice to increase the amount of non-linear emphasis in order to prevent overmodulation, which may cause known inverted white peaks or the like.
If it is desired to further increase the amount of non-linear emphasis, it is a simple matter to set the factor K of the multiplication-by-factor circuit 18 to a greater value.
However, in a system which has a low S/N ratio and in which deterioration in the high-frequency component may easily occur, for example, in the above-described electromagnetic conversion system including the magnetic head, the magnetic disc or the like, if the factor K of non-linear emphasis is set to a large value, the following problems will be encountered. A malfunction may easily occur in the non-linear de-emphasis circuit 14 from the influence of random noise produced during recording or reproduction, with the result that waveform distortion which is unstable with respect to time may easily occur in recovered signals. For this reason, since the value of the factor K is limited, it is difficult to achieve a great effect of improving the S/N ratio by non-linear emphasis.
Such a malfunction in the non-linear emphasis circuit 3 will be described below with reference to FIGS. 9(a) to 9(c).
FIG. 9(a) is a diagram showing the waveform of the input signal of the non-linear emphasis circuit 3, that is, the waveform of a luminance signal having a low S/N ratio which may be input during dubbing. It is assumed that the illustrated waveform is a stair-step waveform which contains random noise. FIG. 9(b) is a diagram showing the waveform of the output signal of the compressor circuit 17 in the non-linear emphasis circuit 3. FIG. 9(c) is a diagram showing the waveform of the output signal of the non-linear emphasis circuit 3.
When a waveform signal such as that shown in FIG. 9(a) is input to the non-linear emphasis circuit 3, the compressor circuit 17 in the non-linear emphasis circuit 3 outputs a differentiated-pulse waveform signal. As shown in FIG. 19(b), although the differentiated pulses generated during a predetermined time interval T should be originally equal in level, they become unequal by the influence of random noise.
More specifically, since the input signal supplied to the high-pass filter (HPF) 16 contains random noise, the differentiated-pulse waveform output from the high-pass filter (HPF) 16 assumes a signal waveform some portion of which increases or decreases in level due to the random noise and another portion of which does not increase or decrease in level since no noise component is produced. Such a signal is applied to the compressor circuit 17 and subjected to non-linear processing for affecting non-linear level suppression according to the level of a signal input to the compressor circuit 17. In the non-linear processing, an input signal whose level is smaller than a predetermined signal level is passed through the compressor circuit 17 without being processed, but an input signal whose level is larger than the predetermined signal level is suppressed by the compressor circuit 17. As a result, if fluctuations in level occur, due to random noise, in a differentiated pulse signal which is a signal input to the compressor circuit 17, it follows that the fluctuations in level occur in a corresponding differentiated pulse output from the compressor circuit 17.
As shown in FIG. 3, in the non-linear emphasis circuit 3, an input luminance signal is supplied to one input terminal of the adder 19, while a signal which is output from the compressor circuit 17 and which is multiplied by the factor K in the multiplication-by-factor circuit 18 is supplied to another input terminal of the adder 19. The adder 19 adds the former signal to the latter signal and outputs the result, whereby non-linear emphasis is affected.
In the non-linear emphasis circuit described above, if the high-frequency component signal output from the compressor circuit 17 fluctuates in level due to random noise, an error may be produced due to such level fluctuations in the process of the addition of the high-frequency component signal to the input luminance signal by the adder 19. As a result, as shown in FIG. 9(c), distortion will occur in the waveform after non-linear emphasis and, for example, the level of each spike pulse increases to an excessive degree or the waveform is partially rounded.
The degree of deterioration in the input luminance signal during non-linear emphasis due to the above-described random noise depends upon the value of the factor K used in the multiplication-by-factor circuit 18. More specifically, the larger the value of the factor K, the more the input luminance signal deteriorates when random noise occurs.
For the above reasons, in such a conventional emphasis circuit, it has been impossible to increase the value of the factor K which is used for multiplication by the multiplication-by-factor circuit 18, that is, the amount of non-linear emphasis. Accordingly, it has been difficult to enhance the S/N ratio.
Also, the above-described apparatus for recording and reproducing still image signals is provided with a dubbing function as an indispensable feature. In dubbing, image signals recorded on one recording medium have been reproduced at least once, and such image signals are again recorded on another recording medium. As a result, random noise which occurs during recording or reproduction causes a malfunction in the compressor circuit of the non-linear emphasis circuit incorporated in the still-image-signal recording and reproducing apparatus which serves as a receiving means. Accordingly, image signals having distorted waveforms are recorded on the recording medium, so that the image signals materially deteriorate each time one dubbing operation is carried out.